Epoxy resin powder coating compositions which are capable of being crosslinked by a variety of agents which effect such crosslinking are well known. The prior art contains numerous references to compositions, which contain epoxy resins and crosslinking agents such as amine, acid anhydrides, boron trifluoride complexes, polyaminoamides, dicyandiamide or substituted dicyandiamides. However, these powder coating compositions yield cured coatings having high gloss. For certain decorative applications, it is desirable to have powder coating compositions that upon curing produce low or semi-gloss finishes that avoid unsuitable light reflections.
It is known that matt finishes can be produced from powder coating compositions in which the binder, pigment, and a matting agent such as micro-fine silicic acid or talcum have been dry-blended. But differences in specific gravity of the dry-blended components, as well as differences in the pick-up by the surface to be coated cause unacceptably large variations in the gloss level of the cured coating.
In U.S. Pat. No. 3,842,035, it was found that a coating having a matt finish can be obtained by using a powder coating composition which is a mixture of two separately prepared heat curable, thermosetting powder coating compositions. One is a slow curing composition and the other is a fast curing composition. The binder components of the two compositions are compatible. When each of the two powder coating compositions is applied separately and cured, a glossy coating results. With this approach, no matting agent is required, but other problems exist, such as dry-blending large amounts of finished powders and the composition of the over spray.
German Pat. No. 2,324,696 teaches a method for making matt coatings by applying to a surface a finely divided mixture of a solid polyepoxide resin and at least one salt of a polycarboxylic acid with three or more carboxyl groups, such as pyromellitic acid, and cyclic amidines, such as 2-phenylimidazoline, and curing the mixture at a temperature of 160.degree. to 240.degree. C.
It has been found that matt finishes having good mar resistance can be produced from epoxy resin powder coating compositions containing an adduct of rosin and maleic anhydride.